Phenylketonuria or PKU has an important place in medical history as the first congenital metabolic disorder. It is a genetic defect in the phenylalanine hydroxylase enzyme existing in the liver and kidneys which is responsible for the conversion of phenylalanine to tyrosine. Deficiency of the enzyme results in accumulation of phenylalanine and its metabolites in the blood and other tissues. If left untreated, mental retardation, speech delays, eczema, seizures, behavioral abnormalities, etc would be expected. In PKU, proper nutrition is the only treatment of choice to prevent complications. The treatment is based on a low-protein diet, and elimination of all protein-rich foods to prevent severe mental retardation. Dietary changes in PKU are permanent through the life. By controlling the diet, we reach disease control too. The metabolic control in childhood and adolescence is related to patients’ quality of life, and their mental status. Even in patients who resume treatment after a period of free diet, an enhancement in the quality of life is observed correlated with the phenylalanine level, in a way that when the phenylalanine level is between 2 and 6 mg/dL, the patients would have a higher quality of life. Moreover, the higher the patients’ phenylalanine level, the lower the quality of life scores. Similarly, phenylalanine levels are associated with patients’ social skills. The lower is the patients’ phenylalanine level, the higher would be the social skills.